Fetal complications, such as low or high birthweight (LBW/HBW), have been significantly associated with later neuropsychiatric outcomes, particularly cognitive abilities, and with medical outcomes, such as cardiovascular disease (CVD). There is a growing recognition that ?normal? variation in fetal growth may also be associated with such outcomes. Past research has been limited by incomplete control over potential confounders that may influence the relationship of fetal measures to adult health, relatively short follow-up periods, and limited access to prospectively collected blood samples with which to measure important precursors of normal development and disease (e.g. hormones). To better establish the association between birthweight and later health and cognitive capacity, we propose a longitudinal sibling-pair design, in addition to LBW and HBW single individuals, from two birth cohorts, which will contain refined measures of these parameters from the fetal period and adulthood. This study is part of a Program Project that will investigate the relationship between fetal growth and adult neuropsychiatric outcomes in tandem with CVD and breast cancer risk. Our primary aims are two-fold: 1) to test for the association between birthweight and adult cognition, psychiatric disorder, and social functioning attainment, and examine the role of fetal growth restriction and mid-gestational maternal hormones (e.g. thyroid) in these relationships: and 2) to examine birthweight associations with well-established neuropsychiatric risk factors for CVD (i.e., personality traits (anger/hostility) and mood) to further examine whether fetal antecedents to certain physiological functions (e.g. autonomic nervous system measures) play a role in both domains. We will use data derived from the New England cohort of the National Collaborative Perinatal Project and the Child Health and Development Study from California, which have remarkably rich and comparable developmental data and archived sera from pregnancy. One thousand same-sex sibling sets (2000 individuals) will be assessed from these cohorts at ages 36-44. We propose to evaluate these subjects in adulthood using clinical and cognitive assessments, to relate adult neuropsychiatric outcomes to fetal growth parameters. The use of sibling pairs will allow for the control of potential familial confounding factors, in particular, socioeconomic status, while assessing the sequelae of fetal growth parameters within sibships and across families. Identifying early determinants of adult health, as proposed here, could significantly aid in preventing or ameliorating disease or disability.